1. Field of the Invention
The present invention relates to an antenna apparatus for carrying out non-contact data communication with a tag provided with an antenna and an IC chip including a memory through an induction field, and particularly to a gate type antenna apparatus in which two antennas having the same shape are arranged to be opposite to each other.
2. Description of the Related Art
A non-contact RF tag is formed of a coil antenna and an IC chip having a built-in nonvolatile memory, and has attracted attention as a recognition technique substituting for a bar code in a system in which data communication with an antenna apparatus generating an induction field is carried out by wireless. In the non-contact tag as stated above, since a power supply for an IC chip operation is extracted from the induction field generated by the antenna, a battery is not needed.
A loop antenna is used for the antenna apparatus, and various antenna shapes are formed according to its use methods and places.
A gate type antenna apparatus in which two antennas having the same shape are arranged to be opposite to each other has been used for an automatic read apparatus at a conventional security gate for preventing shoplifting or at conveyer transfer, or for a read apparatus for warehousing and shipping management of articles, or the like.
The gate type antenna apparatus used for such objects is requested to read the non-contact RF tag in a detection zone, which is interposed between gates, independent of its position and direction, and it is necessary to realize this by a restricted radio wave output within radio law regulations. In order to satisfy the contradictory requests, in a conventional shoplifting preventing system using a radio wave, various configurations of gate type antennas have been proposed. For example, an antenna 100 called a figure-of-eight type antenna shown in FIG. 6 has a configuration including two loop antennas 101 and 102 having the same shape on the same plane, and by feeding reverse phase currents (currents flow in the directions of arrows) to the respective loops, electromagnetic fields can be approximatively cancelled out at a remote place which becomes problematic in the radio wave regulations, and by this, an output higher than that of a single loop antenna can be injected, and consequently, the induction field of the detection zone in the vicinity of the antenna can be intensified. Besides, in U.S. Pat. No. 4,243,980 and U.S. Pat. No. 4,866,455, an expansion type antenna of such a figure-of-eight type antenna is disclosed.
However, in the case where the foregoing figure-of-eight type antenna configuration or its expansion type is applied to the gate type antenna apparatus of the non-contact RF tag, a non-detection region due to the reverse phase currents comes into existence widely according to the direction of the non-contact RF tag, and there is a problem that the request can not be sufficiently satisfied. That is, as shown in FIG. 7, in the case where the non-contact RF tag 103 is opposite to the plane of the antenna 100, the outputs of radio waves are cancelled by an intersecting portion of the antenna, and a non-detection region 104 comes into existence. However, since the conventional tag for preventing shoplifting does not include an IC chip and merely has a function of detecting the existence of the coil, as compared with the non-contact tag requiring an output higher than a certain intensity so as to operate the IC chip, the detection sensitivity is very high, the non-detection region is very narrow, and a problem in practical use does not arise.
The present invention has been made to solve the foregoing problems of the prior art, and an object of the invention is to provide an antenna apparatus in which a non-detection region is made minimum within the regulatory limit of radio wave radiation, and satisfactory communication over a wide range is enabled.
In order to achieve the above object, according to the present invention, an antenna apparatus comprises a first antenna assembly and a second antenna assembly arranged to be opposite to each other, and carries out non-contact data communication with a data carrier, and is characterized in that each of the first and the second antenna assemblies includes a first, a second, a third, and a fourth antennas arranged to be adjacent to each other on a substantially same plane, and the antenna apparatus comprises control means for controlling so that currents flowing to the first antenna and the third antenna have phases reverse to each other, currents flowing to the second antenna and the fourth antenna have phases reverse to each other, and the phases of the currents flowing to the first antenna and the third antenna, or to the second antenna and the fourth antenna are inverted at a predetermined timing corresponding to the data communication.
As stated above, since the reverse phase currents are always made to flow between the first and the third antennas and between the second and the fourth antennas, similarly to the conventional figure-of-eight type antenna, radio wave radiation at a remote place becomes minimum, and the intensity of an induction field in the vicinity of the antenna can be intensified. Besides, since the phases of the currents flowing to the first antenna and the third antenna, or to the second antenna and the fourth antenna are inverted at the predetermined timing corresponding to the data communication, a non-detection region coming into existence in the vicinity of a position where the antennas intersect with each other is complemented in time and the non-detection region can be removed.
Besides, it is preferable to provide inductance addition means for adding an inductance equal to a mutual inductance between the second antenna and the third antenna.
By doing so, mutual couplings between antennas, which occur in the case where the four antennas are arranged adjacently, are cancelled, and power feeding to the antenna apparatus can be efficiently carried out in any phase condition.
Besides, it is preferable that the control means provides a predetermined phase difference between the first antenna assembly and the second antenna assembly.
By doing so, a rotating induction field can be generated in a region between the first and the second antenna assemblies arranged to be opposite to each other, and communication with data carriers existing in all directions in this region can be carried out.
Besides, each of the first, the second, the third, and the fourth antennas is a substantially rectangular loop antenna, and they may be arranged so that sides of adjacent rectangles become substantially parallel to each other.